1. Field of the Invention
The present invention relates to an ion-conductor for a lithium secondary battery and a lithium secondary battery using the same. More particularly, the invention relates to an ion-conductor, to which a specific additive is added, for a lithium metal secondary battery, a lithium alloy negative electrode secondary battery or a lithium ion secondary battery.
2. Related Art
In recent years, primary batteries using lithium as a negative electrode have been put in practical applications, attracting attentions because of their high voltage (3V to 4V) and high energy density. Lithium secondary batteries have also been receiving attentions, because the lithium secondary batteries are promising which will meet portability requirements for personal computers, word-processors, handy phones and the like.
Though the lithium secondary battery has come to spread rapidly in recent years, there is still a problem such that the battery cannot ensure a good charge-discharge efficiency and cycle lifetime. Therefore, there has been a longing for development of an ion-conductor which can provide a battery with improved cycle characteristics such as a charge-discharge efficiency and a cycle lifetime and with improved properties at a low temperature.
As the ion-conductor for a lithium secondary battery, an electrolytic solution form containing an electrolyte and an organic solvent and a solid electrolyte form are known.
As the organic solvent which is contained in the electrolytic solution, a solvent with low viscosity and a solvent with high dielectric constant are known. The charge-discharge efficiency of the battery obtained by using only a solvent with low viscosity is low, so that mixed solvents containing a solvent with low viscosity and a solvent with high dielectric constant are generally used.
As the mixed solvents, for example, conventionally known are a mixed solvent of propylene carbonate (PC) and 1,2-dimethoxyethane (DME), a mixed solvent of PC and benzene or toluene (Japanese Unexamined Patent Publication No. SHO 52 (1977)-109116), a mixed solvent of PC and tetrahydrofuran (Japanese Unexamined Patent Publication No. SHO 57 (1982)-118375), a mixed solvent of PC and dimethyl carbonate (DMC) or diethyl carbonate (DEC) (Japanese Unexamined Patent Publications No. HEI 2 (1990)-148665 and No. HEI 4 (1992)-171674) and the like. Also a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) or diethyl carbonate (DEC) (Japanese Unexamined Patent Publication No. SHO 59 (1984)-96666) is known.
As the solid electrolyte, polyethylene oxide (PEO) and the like are known.
In the case of a lithium metal secondary battery using lithium or an alloy thereof as a negative electrode active material, there has been a problem described below if an electrolytic solution containing an organic solvent is used.
That is, the organic solvent and the negative electrode active material react on a negative electrode side of the lithium metal secondary battery while on a positive electrode side thereof the electrolytic solution tends to be decomposed as a result of an electric potential kept at a high voltage. Therefore, there has been a problem such that the battery can not ensure good cycle characteristics.
In the case of a lithium ion secondary battery using a carbon material (e.g., graphite) as a negative electrode active material, there has been a problem as described below if an electrolytic solution containing an organic solvent composed of PC is used.
Namely, though it is preferable to use PC because PC is capable of keeping good low-temperature characteristics, the battery using PC can not ensure good cycle characteristics because PC tends to be decomposed during charging processes. Further, because there is a decomposed product inside the battery, there is a problem such that a danger of explosion or the like increases.
In order to solve this problem, EC can be used in place of PC. However, a melting point of EC is as low as 37 to 39.degree. C., so that there arises another problem such as deterioration of cycle characteristics of a lithium ion secondary battery at low temperatures.
With respect to a solid electrolyte, a lithium secondary battery using the solid electrolyte has a disadvantage such as deterioration of cyclic characteristics because of low conductance thereof.
As mentioned above, the conventional ion-conductors for lithium secondary batteries are not stable enough for any application to the lithium metal secondary battery or the lithium ion secondary battery. Moreover, the conventional ion-conductors can not provide a lithium secondary battery with satisfactory cycle characteristics such as charge-discharge efficiency, cycle lifetime and the like.